Facile Syntheses of N-Heterocyclic Carbene Precursors through I2- or NIS-Promoted Amidiniumation of N-Alkenyl Formamidines

We have developed I-2- or N-iodosuccinimide (NIS)-mediated amidiniumation of N-alkenyl formamidines for the syntheses of cyclic formamidinium salts, some of which could be directly used as N-heterocyclic carbene (NHC) precursors. Treatment of iodine-containing formamidinium salts with Al2O3 led to the formation of cyclic formamidinium salts with an unsaturated backbone. A rhodium(I) complex ligated by a representative NHC was prepared by the reaction of [Rh(cod)Cl](2) (cod= 1,5-cyclooctadiene) with the free carbene obtained in situ from deprotonation of the corresponding formamidinium salts. The NHCs prepared in situ can also react with S-8 to afford the corresponding thiones.